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Old 01-05-2015, 03:37   #1
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Use a normal car battery for solar panel?

Hi again,

I am wondering if it would be fine to use a Solar Panel to charge a normal car battery?

As the last owner of my boat hasn't installed any AGM/GEL batteries, just simple car batteries. Upon sailing the boat back, I don't want to do any major installations, but still want to cruise a bit with the comfort of being able to charge the batteries under way.

My charger-controller has 2 options:
Lead-acid batteries with liquid electrolyte
Sealed lead-acid batteries; AGM, GEL

I suppose a regular car battery would fall under "Lead-acid batteries with liquid electrolyte"?
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Old 01-05-2015, 05:06   #2
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Re: Use a normal car battery for solar panel?

Yes, a standard automotive starting battery is more likely a lead acid battery with liquid electrolyte, commonly called FLA (flooded lead acid) battery.

Why is it that you want to buy an AGM? What benefit are you looking for? Are you aware that AGM batteries can die an early death if not fully charged on a regular basis? Since they are rather exepensive that could be a painful lesson.

If you have plenty of solar to keep the battery topped off or keep the boat plugged into shore power and a charger on this could be fine but if you keep the boat off the grid without a power source could be a problem.

Generally the best bang for buck is a deep cycle battery. A pair of 6V golf cart batteries in series is a great option. I have six of them.
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Old 01-05-2015, 05:12   #3
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Re: Use a normal car battery for solar panel?

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Yes, a standard automotive starting battery is more likely a lead acid battery with liquid electrolyte, commonly called FLA (flooded lead acid) battery.

Why is it that you want to buy an AGM? What benefit are you looking for? Are you aware that AGM batteries can die an early death if not fully charged on a regular basis? Since they are rather exepensive that could be a painful lesson.

If you have plenty of solar to keep the battery topped off or keep the boat plugged into shore power and a charger on this could be fine but if you keep the boat off the grid without a power source could be a problem.

Generally the best bang for buck is a deep cycle battery. A pair of 6V golf cart batteries in series is a great option. I have six of them.
Perhaps I should have mentioned the boat is a tiny 27 foot sailboat without any real power draws. There's a radio and lights, that's about it.

I calculated my daily power usage to be around 100-200 watt.

My solar panel is a 100watt panel so it should be more than enough to cover my daily usage.

The idea to buy an additional AGM battery, is simply because I heard so much bad about discharging a car battery more than 90%. And if want to run the radio for a couple of hours in the evening + lights, I'm just afraid that i will drain the battery too much and ruin it.

Maybe I'm worrying too much?
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Old 01-05-2015, 05:19   #4
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Re: Use a normal car battery for solar panel?

Probaly OK

There are two main types of batteries: Start and Deep Cycle.
Both come in all sorts of types, like liquid acid, AGM and Gel.

The Start battery is made for large, but short power load. This is measured in Cold Cranking Amps (CCA).

Deep Cycle is good for long term loads and is good down to 50% of capacity (measured in Amp-hours.

SO for your needs, it helps to know what kind of battery you have. It's usually written on the side or top.

Hope this helps.
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Old 01-05-2015, 05:21   #5
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Re: Use a normal car battery for solar panel?

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Probaly OK

There are two main types of batteries: Start and Deep Cycle.
Both come in all sorts of types, like liquid acid, AGM and Gel.

The Start battery is made for large, but short power load. This is measured in Cold Cranking Amps (CCA).

Deep Cycle is good for long term loads and is good down to 50% of capacity (measured in Amp-hours.

SO for your needs, it helps to know what kind of battery you have. It's usually written on the side or top.

Hope this helps.
This is pretty much the only info I have on the batteries until I get to pick up the boat at the end of the month:

One battery is 75 amp and the other one is 65 amp. It's ordinary car batteries with water.

I'm not sure if that gives you any useful info?
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Old 01-05-2015, 05:26   #6
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Re: Use a normal car battery for solar panel?

Well when you get to the boat just look.

Your 200 watts = 200 amps x volts, so 200 amps x volts /12 volts = 18 amps.
So you should be fine, just make sure what type of battery, Deep Cycle or Start.
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Old 01-05-2015, 05:46   #7
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Re: Use a normal car battery for solar panel?

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Originally Posted by Dauntlessny View Post
Well when you get to the boat just look.

Your 200 watts = 200 amps x volts, so 200 amps x volts /12 volts = 18 amps.
So you should be fine, just make sure what type of battery, Deep Cycle or Start.
Since it's car batteries aren't they all Start?

I can only assume it's the cheaper version to be honest (which I guess would be start?)

Does it make any difference when running my 12v items if its start/deep cycle?
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Old 01-05-2015, 06:02   #8
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Re: Use a normal car battery for solar panel?

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Since it's car batteries aren't they all Start?
All? No. Most? Yes.

Most "car batteries" are starting batteries. But the terminology that you are using is clouding the issue. It appears that you, and the previous owner, are making the assumption that because it is not an AGM or Gel battery--because it is "with water"--that it must be an ordinary, car starting battery. That is not necessarily so.

There are deep cycle batteries that take water and look just like ordinary starting batteries. That is why people have suggested that you take a good, close look at the batteries when you get a chance. They may be deep cycle batteries, which are better for your usage than a starting battery (even though a starting battery will work just fine--it just is not likely to last as long).

When it comes time to replace these batteries it is not necessary to replace them with AGM or Gel batteries. In fact, as others have mentioned, there may be good reasons not to. But whatever type of battery you get, you want to be sure that it is rated for deep cycle use.

Hope that helps.
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Old 01-05-2015, 06:05   #9
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Re: Use a normal car battery for solar panel?

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Hope that helps.
It sure does, thank you!

So there is no need for me to worry about the batteries not lasting the 2 weeks sailing the boat home when I have a 100watt solar panel connected to continuesly charge up the batteries during use?

(Both batteries are new btw.)
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Old 01-05-2015, 06:15   #10
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Re: Use a normal car battery for solar panel?

I think you will be fine, but if you don't have a volt meter, I'd buy one, probably less than $5 on Ebay etc. I'd not discharge them below 12V if they have to start an engine in the morning.
This is Rube Goldburg battery management, 12V under even a small load is not 50% discharged, there is a fudge factor that should keep you safe.

Lights and just a radio? I don't think I'd honestly go any further than Walmart car batteries for that. Nothing wrong with KISS
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Old 01-05-2015, 06:33   #11
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Re: Use a normal car battery for solar panel?

You are probably fine.

Yes, it is likely the cheapest starting battery they could find.

Starting and deep cycle batteries can be used for either starting or house loads without fear. It's just that they are more effective at thier stated purpose. It takes a smaller starting battery to start the engine and a smaller deep cycle to maintain house loads but no harm will come from using either for the other purpose as long as you don't drain them too low (try not to go below 50% charge and make sure they are fully charged periodically).

200watts doesn't tell us what we need to know. Batteries are rated in amp-hrs. You need to estimate how many amps you draw and how long those draws will be. If a device is rated in watts, divide by the voltage to get the amps. Example: if you have a 24watt light bulb it will draw 2 amps and if it's on for 6 hrs, it will use 12 amp-hrs.

Go thru and estimate how long each device will be on while you are cruising and total up your amp-hrs. If it's more than 30-50% of the batteries rated amp-hrs, you are likely to have issues. If excessive, you will wind up with a dead battery but even if you are drawing the battery way down but not dead, you will experience a short battery life before it fails.
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Old 01-05-2015, 06:39   #12
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Re: Use a normal car battery for solar panel?

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You are probably fine.

Yes, it is likely the cheapest starting battery they could find.

Starting and deep cycle batteries can be used for either starting or house loads without fear. It's just that they are more effective at thier stated purpose. It takes a smaller starting battery to start the engine and a smaller deep cycle to maintain house loads but no harm will come from using either for the other purpose as long as you don't drain them too low (try not to go below 50% charge and make sure they are fully charged periodically).

200watts doesn't tell us what we need to know. Batteries are rated in amp-hrs. You need to estimate how many amps you draw and how long those draws will be. If a device is rated in watts, divide by the voltage to get the amps. Example: if you have a 24watt light bulb it will draw 2 amps and if it's on for 6 hrs, it will use 12 amp-hrs.

Go thru and estimate how long each device will be on while you are cruising and total up your amp-hrs. If it's more than 30-50% of the batteries rated amp-hrs, you are likely to have issues. If excessive, you will wind up with a dead battery but even if you are drawing the battery way down but not dead, you will experience a short battery life before it fails.
Thank you for your answer.

Sorry, I calculate in watt, perhaps I should calc in amp hours

What I meant, was my total power consumption during a day will be around 200watts (like using a 10watt item for 20 hours).

I am just looking around at some youtube videos for my charge controller and I am a bit confused. I was under the assumption that a reading of 12,7 volt would mean that my battery was at 100% - however, in some videos (like here: ) he browses through the options (around 1:30 mark) and it shows his battery is 61% state of charge, but when going to the next display it shows 13,5v - which should be well above 100% ?? (When 12,7v should be 100%?) Am I missing something here?
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Old 01-05-2015, 06:48   #13
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Re: Use a normal car battery for solar panel?

Battery management is very complicated, and can get very expensive too. I believe you are confusing resting voltage with charging voltage.
For example, as long as your driving your car, you will see 14V plus or minus a little, battery won't be at 14 the next day before you start, that's resting voltage.
amps is what most people use, but personally I'd rather see Watts as I believe they are more accurate because a Watt is a Watt, amps vary with voltage.
Your consumption based on 200W should be about 15 to 16 amp hours, and two car batteries ought to handle that easily.
Is the Solar panel the only charge source? No motor and alternator?
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Old 01-05-2015, 06:56   #14
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Re: Use a normal car battery for solar panel?

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Battery management is very complicated, and can get very expensive too. I believe you are confusing resting voltage with charging voltage.
For example, as long as your driving your car, you will see 14V plus or minus a little, battery won't be at 14 the next day before you start, that's resting voltage.
amps is what most people use, but personally I'd rather see Watts as I believe they are more accurate because a Watt is a Watt, amps vary with voltage.
Your consumption based on 200W should be about 15 to 16 amp hours, and two car batteries ought to handle that easily.
Is the Solar panel the only charge source? No motor and alternator?
Haha, yes I probably mixed up those two. I thought the display was a volt meter to display the current voltage - but I guess it only displays that in percentages, which is also fine. Just got confused

There is an inboard MD7A with an alternator, so that would also be able to charge the batteries, but I would prefer not to run the engine just for charging up the batteries.

For that reason I would like to try and rely mostly on the solar panel.

I don't think I will consume any more than 1-3 amps simultaneously during the day, so my 100 watt solar panel should hopefully easily keep the battery topped up - and then it's just a little usage during the evening that will draw on the battery.
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Old 01-05-2015, 06:58   #15
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Re: Use a normal car battery for solar panel?

Not trying to be snarky but watts are an instantainious rate of power consumption. Using 200watts per day doesn't have any meaning and can lead to confusion if people make assumptions (If I took your initial posting literally, it would imply a 200watt draw 24hrs a day, which would be a very large draw for a small simple boat)

You could do watt-hrs but since batteries are rated in amp-hrs, the common usage is to calcualte your consumption in amp-hrs. If you want to work in watt-hrs, multiply the rated amp-hrs of the battery by 12 to get watt-hrs.

It is true that battery voltage is not a constant but increases as the battery reaches full charge. Also while charging it will often read a couple volts higher than when you take the charging source off it will drop back over the next hour or two. For estimating purposes, assuming 12V is generally close enough.

I don't remember off the top of my head but google, battery voltage charge state and you will get the correlation between voltage and charge state (take the battery off all charge sources for a while before checking to get an accurate reading.
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